Field of the Invention
The present invention relates to power supply control of an electronic device.
Description of Related Art
Technology is known in which a USB (universal serial bus) cable is conventionally used to charge batteries, supply power to an electronic device (system), or to transmit data by connecting a PC (personal computer) to an electronic device having an embedded battery (secondary battery) capable of being charged, such as a digital camera, tablet terminal, or smartphone.
Technology is also known in which a digital camera and PC are connected by a USB cable, and the current supplied from the PC minus the amount of current needed for data transmission with the PC is used to charge the batteries, for example.
Furthermore, if the digital camera is constituted of a plurality of functional blocks and connected by a USB cable to the PC, then the power capable of being supplied from the PC is confirmed, and the supplied power is allocated to prescribed functional blocks on the basis of these confirmation results.
The amount of current that can be supplied to the electronic device from the USB terminal of the PC, however, is limited to a maximum of 500 mA or the like, for example. FIG. 8 is a conceptual view of one example of power being supplied to an electronic device by a USB cable using conventional technology. In FIG. 8, the horizontal axis is time and the vertical axis is current supplied from the PC, or namely the current flowing to the USB cable. When the USB cable is connected, from time t0 to t1 a current that changes in accordance with load variation is continually supplied to the system section (electronic circuits, driving section, and the like), and excess current out of the 500 mA is supplied to the battery.
At time t1, the battery has been charged to a certain degree and the charging current is reduced, and then further reduced the closer the battery is to being fully charged. At time t6, the battery is fully charged and the charging stops (the charging current becomes zero), and only the system current remains. In other words, the current of the USB cable changes as shown by a solid line L1 in the drawing.
In this manner, if the sum of the current and charging current supplied to the system section in conventional technology is less than or equal to the current 500 mA usable by the USB terminal (after time t1), then changes in the current or the charging current after load fluctuations of the system section will appear as a change in the current flowing through the USB terminal and the USB cable, as shown by the solid line L1 in FIG. 8. In particular, the current has relatively large fluctuations when a zoom lens or focus lens in the system section is driven by a motor, as shown by still image importing in times t2 to t3 or live image importing in times t4 to t5, or when high-grade image processing is performed. The changes in current caused by these load fluctuations in the system section also change the current flowing to the USB cable, and thus, the USB cable becomes a source of EMI (electro-magnetic interference) and noise.